With the growing demand for ever greater miniaturization of ULSI devices, planarization via CMP becomes an increasingly critical aspect in the fabrication sequence of semiconductor devices. The challenge stems, inter alia, from the multitude and differing nature of materials used in the various layers, the demanding geometries and aspect ratios of the structures, the ever present quest for improved IC device flatness and better yields via reduction of defects.
Broadly, there are known two types of CMP compositions and processes:
A. Slurry-based CMP, wherein abrasive particles contained in an aqueous suspension along with a host of other ingredients are delivered onto a pad, typically made of polyurethane, or polyuretahane composites, with the surface to be planarized rubbing against the rotating pad, resulting in levelling action via removal of protruding/uneven matter.
B. “Fixed abrasive” pads, wherein abrasive particles are embedded in a binder, and, as a rule, need not to be delivered, separately. The polishing pad generally constitutes the upper portion of a three layered construction.
The fixed abrasive technology appears to be gaining momentum, for some of the reasons listed below:
1. Slurry-based systems can be prone to inconsistent and uneven slurry distribution across the polishing pad, leading to unsatisfactory planarity in the polished substrate.
2. Slurry suspensions tend to settle and become less than homogeneous, again causing uneven polishing action.
3. Slurry suspensions can clog up delivery ducts and apparatus, requiring somewhat taxing cleaning and maintenance regimes.
4. Surfaces and pores of polishing pads tend to deteriorate as a result of hydrolytic exposure of the polyurethane surface to the slurry suspension, resulting in inconsistent performance.
5. Slurry-based polishing pads and systems tend to generate waste and are less than environmentally friendly.
Above enumerated shortcomings of slurry-based CMP systems, are generally not encountered in fixed abrasive polishing constructions, wherein the abrading layer is encapsulated in a binder, and engineered to achieve maximum flatness, that generally duplicates similar flatness or planarity in the wafer that is to be polished.
However, as is generally the case with technical improvements, some of the benefits of “older, i.e. slurry-based pads and systems, are lost in the fixed abrasive constructions. Still, fixed abrasive, precision-engineered pads and methods are the preferred choice in many instances in operations where maximum planarity is key.
Perhaps the most salient benefit missing in fixed abrasive elements, is that slurry-based compositions can be formulated to contain, in addition to the abrasive particles, other valuable chemical components, such as wetters, oxidants, leveling agents and the like, making the slurry suspension self-sufficient throughout the polishing operation, because the slurry composition contains all that is chemically required for synergistic interaction of mechanical abrasion coupled with chemical interaction at the slurry/wafer interface. Indeed, in the case of fixed abrasive pads on the market today, needed chemicals i.e. oxidants, must be delivered separately.
A. Related Art on Slurry-Based CMP Compositions
Patent applications WO 02/083804 to Costas, US 2002/0177316 A1 to Miller and WO 01/44396 A1 to Sachan, are referenced herewith as indicative of methods and compositions of typical slurry-based CMPs of the prior art. They reflect the differing natures of CMP compositions, dictated by the tasks/problems they need to address, for example nature of the layers, selectivity, surface roughness and throughput.
CMP slurries can be somewhat simplistically described as consisting of abrasive particulate matter suspended in aq., desirably stable, compositions. Such suspensions usually contain a host of additives, pH adjusters, leveling agents, emulsifiers, and the like. In slurry-based CMPs, the slurry is usually dispensed on a rotating pad in contact with a rotating wafer. Planarization is said to involve a combination of abrasion, as well as chemical reaction at the wafer/slurry interface.
A significant, and generally central component of various slurry-based chemical mechanical polishing systems, is the oxidizing agent, typically H2O2. The choice of the oxidizing agent is usually tailored to suit a given substrate to be polished, with copper perhaps being the most challenging, as it is becoming the metal of choice for interconnect applications, due to its superior electrical conductivity.
While hydrogen peroxide is an attractive oxidizing agent at reasonable cost, it is not without some serious drawbacks, namely poor stability, especially in the presence of transition metals that are known to catalyze decomposition. Another shortcoming of H2O2 is its less than ideal selectivity. Further, the reaction of peroxides during dissolution of copper, is highly exothermic, making it problematic to maintain temperature stability at the copper/slurry interface, where polishing takes place.
U.S. Pat. No. 6,448,182 to Hall addresses the stability issue of H2O2 through incorporation of stabilizers that are intended to reduce, but will not eliminate, decomposition.
CMP is said to be effected by a dual, said to be synergistic, mechanical/chemical process. The mechanical aspect is obtained by applying downward pressure, with the abrasive in the slurry removing unwanted surface material. As such, the mechanism of the abrasive action is relatively simple and fairly well understood. On the other hand, the chemical mechanism of CMP is more complex, and its interaction with the mechanical component of CMP has yet to be fully understood, namely as to how it participates in promoting the desired final surface finish, namely smoothness, specularity, freedom from oxides, and the like. In the case of copper conductors, the chemical aspect of CMP is significant, indeed. Hence, the crucial importance of oxidants.
B. Related Art on Fixed Abrasive Pads
Reference is made to U.S. Pat. No. 5,692,950 to Rutherford, disclosing a fixed abrasive polishing construction, and a method to manufacture it, as detailed in Example 1 of the disclosure. It is noted that the fixed abrasive layer, the one that effects grinding or material removal, is produced by blending a slurry comprising cerium oxide and calcium carbonate, with acrylates, plasticizers, coupling agents, photoinitiators, etc. This abrasive layer constitutes the abrading, upper portion of the fixed abrasive assembly. There is no apparent provision in the disclosure as to the method of using the fixed abrasive structure, especially as to whether it is to be used as is, or if polishing is assisted by fluid being dispensed to the pad/wafer interface during abrasion.
U.S. Pat. No. 6,346,032 to Zhang discloses a fluid dispensing arrangement designed to deliver a “variety of fluids” that assist in the polishing process of fixed abrasive polishing pad, and/or waste particle removal from the surface to be polished.
Published PCT application # WO 02/18099 A2 to Chopra, has provision to deliver oxidizing solutions, with or without slurry, to the fixed abrasive pad/wafer interface in order to assist with metals, especially copper, and the barrier layer such as tungsten. Hydrogen peroxide is apparently preferred for copper.
U.S. Pat. No. 6,364,749 to Walker, addresses wetting problems associated with fixed abrasion assemblies, specifically with the outer surface of protruding abrasive particles that are encapsulated in a hydrophobic resin. It is not completely understood how wetting can be preserved as the outer layer of the pad is undergoing wear during polishing.